Furnace gases can flow out through the electrode holes in the roof of an electric arc furnace. When a fourth hole is provided in the roof, problem of flow through an electrode hole is alleviated, but is not solved. The furnace gases can generate flames, which sometimes reach a height of two or more meters. The overflowing gases always contain solid particles of iron oxides and calcium oxides (lime).
The problem of sealing the electrode holes arose with the development of the first electric arc furnaces at the beginning of this century. Different sealing devices have been suggested. All these devices only reduce more or less the outflow of furnace gases, but do not fully prevent it. Despite this drawback, many electric furnaces presently use different designs of such sealing devices.
During the 1970's the wide spread use of sealing devices which operate on the "air cushion" principle commenced. These devices form an annular chamber around the electrode into which such a quantity of air is blown that, during the air outflow upwardly along the electrode, there is maintained in the chamber a pressure slightly higher than that inside the furnace space, i.e. about 3 mm water column. Such sealing devices are made of metal without or with water cooling or of an extremely metallic and an internally refractory chamotte portion. The metallic devices without water cooling are simple to manufacture and seal well, but their life is limited because of deformations and an the increase of the internal hole size. The metallic devices with water cooling are more complicated to manufacture and are more expensive, but they have a life which is several times longer life.
There have been suggested sealing devices of the same type which are provided with an internal chamotte refractory. These devices have a substantial drawback due to the relatively low fire-resistance of the chamotte materials. Depending on their chemical composition, they soften at about 1500.degree. C. As a result, during operation their internal hole size increases gradually; this impairs or even terminates the sealing action.
All known sealing devices operating on the "air cushion" principle are provided for the air delivering with a duct-diffuser with a nozzle along the duct axis. Gas is delivered under pressure through this nozzle. This gas flow entrains in the duct-diffuser a several times greater air volume from the atmosphere. Through a hole in the side wall of the device, the gas-air flow enters the chamber around the electrode and effectively blocks totally the outflow of furnace gases.
According to the specific conditions in a given plant, the pressurized gas can be compressed air, steam, waste industrial nitrogen, or another similar gas. The choice of the pressurized gas is determined by economic considerations. The composition of the gas does not influence the sealing action.
During the last ten years electric arc steelmaking furnaces have undergone many improvements and this has resulted in an increase of their output by two to three times. The endurance of the sealing devices is directly related to the use of water-cooled roofs, to the sharply increased secondary voltage of the transformers, and the pneumatic transport and injection of alloying and slag-forming materials. The metallic sealing devices cannot endure for a sufficiently long time such heavy-duty conditions and, because of the comparatively frequenct arcing by momentary powerful electric arcs between the sealing devices of two adjacent electrical-supply phases, they are quickly damaged. The failure of the sealing devices with internal chamotte bodies occurs very fast because of the insufficient fire-resistance of the chamotte material.